Although highly promising, the use of stem cell-derived cardiomyocytes for treatment of heart disease faces a number of challenges. In particular, therapies involving pluripotent stem cells suffer from the lack of methods to select a desired cardiac electrical phenotype and generate mature cardiomyocytes in vitro, as well as control differentiation fate of transplanted cells and their interactions with host cardiomyocytes n vivo. Together, these limitations can render stem cell-based cardiac therapies not only inefficient, but also tumorigenic and arrhythmogenic. Similarly, recently developed reprogramming techniques to directly convert cardiac fibroblasts to cardiomyocytes suffer from low efficiency and reproducibility, as well as the inability to obtain functional phenotype with human cells even after several weeks of culture. Ideally, a safe and efficient cardiac cell-based therapy should involve the implantation of homogeneous cells or engineered tissue grafts with the functional properties that are stable and similar to those of the surrounding adult cardiomyocytes. Thus, we propose to develop a new bioengineering strategy for cell-based cardiac repair that does not rely on the use of stem cells or direct reprogramming of fibroblasts to cardiomyocytes. Rather, building on our proof-of-concept studies with immortalized human cell lines, we propose to rapidly and efficiently convert primary human dermal fibroblasts into electrically active cells capable of action potential conduction and functional coupling with cardiomyocytes. Specifically, we propose to: 1) develop genetic engineering algorithm to stably convert adult human fibroblasts into electrically conducting cells with tailored electrophysiological phenotype resembling that of neonatal or adult rat cardiomyocytes, and 2) evaluate the therapeutic potential of injected electrically active fibroblasts and tissue patches made of these cells in a rat model of myocardial infarction. Successful competition of the proposed studies will allow us to evaluate the potential of engineered excitable somatic cells for future use in experimental studies in vitro and cell-based cardiac therapies in vivo. The outcomes of this project may also promote the development of new gene therapies for heart disease where selective in situ conversion of endogenous cardiac fibroblasts into electrically excitable and conducting cells could significantly improve compromised heart function.